The present invention relates to high-efficiency coding and decoding apparatus which are used in recording, transfer and display apparatus which perform digital signal processing, and which perform efficient coding and decoding, and in particular, to coding and decoding apparatus which perform inter-image processing of moving image signals have small deterioration of image quality even when there are transmission errors.
High-efficiency coding of moving image signals (moving images) can involve interframe predictive coding which uses the correlation between frames of image signals and uses a frame for which coding has been performed, to predict and code only the prediction error. In recent years, motion compensation predictive coding has become the general method used for prediction in accordance with motion of an image.
On the other hand, in coding in which a storage media is the object, intraframe independent coding is performed without interframe prediction for each of several frames and this enables random access and high-speed search.
In addition, there is also known a method such as MPEG (ISO-IEC) which uses skip prediction and pre- and post-prediction between skip predictions to raise the coding efficiency. With the MPEG method, differences in the method of prediction mean that a frame can be an I (intra) frame coded independently within a frame, a P (Prediction) frame which is skip predicted, or a B (Bi-directional) frame which is pre- and post-predicted.
The following is a description of a detailed configuration of a conventional coding apparatus.
FIG. 1 shows an example of the configuration of a coding apparatus of the MPEG type. Here, the frame types of I, P and B cause the changeover switch 2, 4, 22 to be controlled by sync signals separated from input signals, and to be switched to the positions shown in the figure.
Image signals input from an image input terminal 1 are directly led to a predictive, subtracter 5 via the changeover switches 2 and 4 in the case of I or P frames, while B frames are led to the predictive subtracter 5 after having been delayed until there is pre- and post-I and P in a frame memory 3. In the predictive subtracter 5, prediction signals arriving from an adaptive predictor 42 are subtracted from input signals and a prediction residual signal is output to become coded data compressed by coding in an intraframe encoder 6.
In the intraframe encoder 6, DCT (discrete cosine transform) is first preformed, and that conversion output is quantized, and given a variable length coding such as Huffman coding or the like. That compressed DCT information is applied to a multiplexer 40 and in the case of I and P frames, is led to an intraframe decoder 21 via the changeover switch 22.
The intraframe decoder 21 first decodes the variable length coding, and replaces the fixed-length codes with quantized representative values, and also performs reverse DCT to obtain the reproduced signals. In the intraframe decoder 21, the reproduced prediction error signals have the prediction signals added in a residual adder 20 to produce the reproduced image signals. The reproduced image signals are stored in a frame memory 19 while the signal that have been stored in the frame memory 19 until that time are transferred to a frame memory 18.
The output of the frame memory 19 is given to a motion compensator 15 and a motion vector detector 17, and the output of the frame memory 18 is given to a motion compensator 14 and the motion vector detector 16.
For each block of 16.times.16 picture elements, the motion vector estimators 16 and 17 detect the motion vectors between the input signals and the signals given to the frame memories 18 and 19. The motion vector information is given to the motion compensators 14 and 15 and also to a multiplexer 40. The motion compensators 14 and 15 spatially move reproduced image signals stored in the frame memories 18 and 19 by the motion vector portion given from the motion vector detector, and applies them to an adaptive predictor 42.
For the same block as the motion vector detection, the adaptive predictor 42 creates four types of prediction signals from the two signals (F and B) which have been motion compensated, and of those, the optimum prediction signals is decided from matching with the input signals which become the signals to be predicted.
The prediction mode used here is one of the four types of only the "F" (Front: prediction signals from the frame temporally prior) mode, only the "B" (Back: prediction signals from the frame temporarily later) mode, the "(F+B)/2" mode or the "0" mode, with the "0" mode being intraframe independent coding. The prediction mode is only the "0" mode for I-frames, the "F" and "0" modes for P-frames, or any of the four modes for B-frames.
The multiplexer 40 recombines the DCT information which is the output of the intraframe encoder 6, the prediction mode information (MODE) which is the output of the adaptive predictor 42, the motion vector information (MVF and MVB) which is the output of the motion vector detector 16, for each block (macroblock: MB) for which the motion vector and the prediction mode have been determined, and outputs them via a data output terminal 12, to the side of a decoding apparatus. FIG. 6A shows the configuration of the data. Here, there is no transfer of the motion vector information not used in the prediction.
The following is a description of a conventional decoding apparatus.
FIG. 2 is a view showing the configuration of a decoding apparatus. Those portions which correspond to portions of the coding apparatus of FIG. 1 are shown with corresponding numerals. The coded data which is input from a data input terminal 30 is disassembled into each information by a demultiplexer 41 and the DCT information is applied to the intraframe decoder 21, the prediction mode information is applied to an adaptive predictor 43, and the motion vector information is applied to the motion compensators 14 and 15.
The DCT information is decoded by the intraframe decoder 21, and prediction signals are added at the residual adder 20 to create the reproduced image signals.
In the case of B-frames, reproduced image signals are immediately outputted from a reproduced image signal output terminal 36 via changeover switches 34 and 35, while I- and P-frames are stored in the frame memory 19. The signals which have been stored in the frame memory 19 up till that time are moved to the frame memory 18 and are outputted from the reproduced image signal output terminal 36 via the changeover switch 35.
The output of the frame memory 19 is applied to the motion compensator 15 while the output of the frame memory 18 is applied to the motion compensator 14. The motion compensators 14 and 15 spatially move the reproduction image signals stored in the frame memory, by the motion vector portion given from the demultiplexer 41, and applies them to the adaptive predictor 43. The adaptive predictor 43 makes the prediction signals from the prediction mode information given from the demultiplexer 41 and outputs it to the residual adder 20.
Here, the inter-image processing units are frames but the description is the same if they are fields of interlace signals.
When there is a coding error between the coding apparatus and its decoding apparatus during transfer or recording, normal demodulation does not occur and there is a deterioration in the image quality. Coding errors result in cell loss in ATM (asynchronous transfer mode) circuits when they occur in normal circuits and recording media, and this loss in cell units becomes a "dropout".
In this case, even for the case of the coding apparatus and the decoding apparatus shown as the conventional example, there is normally detection to the effect that a coding error has occurred and so with prediction residual errors, the prediction residue is not added and the reproduced image signals are the prediction signals only, to result in there being no particularly large deterioration. However, there is absolutely no decoding of a block if there is "dropout" of the motion vector, adjacent blocks and the like are used for interpolation within the same frame and there is no image deterioration as a result.
On the other hand, with a coding method which periodically has independent frames, the deterioration stops with the independent frames and so this method appears advantageous at first. However, coding errors in independent frames can only be compensated for spatially and so there the deterioration becomes large, and the image is influenced later. Furthermore, the amount of data for independent frames is larger than that for prediction frames and so when there are ten independent frames at once, the amount of data is about 40% of the overall amount of data, and the influence of coding errors becomes serious.